The controller area network (CAN), and an extension of the CAN known as a Time Triggered CAN (TTCAN), are discussed for example in laid open print DE 100 00 305 A1.
The media access control method used in the CAN is based on a bitwise arbitration. In the bitwise arbitration, a plurality of subscriber stations can simultaneously attempt to transmit data via the channel of the bus system without thereby disturbing the data transmission. During the transmission of a bit via the channel, the subscriber stations can determine the logical state (0 or 1) of the channel. If a value of the sent bit does not correspond to the determined logical state of the channel, the subscriber station then terminates the access to the channel. In CAN, the bitwise arbitration is standardly carried out in an arbitration field within a data frame that is to be transmitted via the channel. After a subscriber station has completely sent the arbitration field on the channel, the station knows that it has exclusive access to the channel. Thus, the end of the transmission of the arbitration field corresponds to a beginning of an enable interval within which the subscriber station can make exclusive use of the channel. According to the protocol specification of the CAN, other subscriber stations are not permitted to access the channel, i.e. send data on the channel, until the sending subscriber station has transmitted a checksum field (CRC field) of the data frame. Thus, an end time of the transmission of the CRC field corresponds to an end of the enable interval.
The bitwise arbitration achieves a non-destructive transmission of the data frame via the channel. This results in good real-time characteristics of the CAN, whereas media access control methods in which the data frame sent by a subscriber station can be destroyed due to a collision with further data frames sent by another station during the transmission via the channel have a significantly less favorable real-time characteristic, because, due to the collision and the new transmission of the data frame that is thus required, there is a delay of the data transmission.
A further improvement of the real-time behavior of the CAN is achieved through the extension TTCAN. According to the TTCAN protocol specification, a time window structure is defined that includes a plurality of successive time windows (often also called time slots) and that repeats regularly. Here, a particular message type, and thus a particular subscriber station, can have assigned to it a particular time window within which messages of this message type may be transmitted. Thus, in TTCAN particular time windows are provided within which a particular station has exclusive access to the channel of a CAN domain. In TTCAN, the access to the channel is coordinated at least partly according to the time-based multiple access design (Time Division Multiple Access, or TDMA).
The CAN protocols, or the protocols of its extension TTCAN, are suitable in particular for transmitting short messages under real-time conditions. If, however, larger data blocks are to be transmitted via a CAN domain, then the relatively low bit rate of the channel may be noticeable in a disturbing manner. In order to ensure the correct functioning of the bitwise arbitration, for the transmission of a bit a least duration must be adhered to that is a function in particular of the extent of the bus system and is also a function of the signal propagation speed on the channel. Thus, the bit rate cannot be easily increased by reducing the duration of the individual bits.
To make it possible to transmit relatively large data quantities with adequate speed via a communication interface parallel to a slower data stream controlled by a media access control method, such as a CAN data stream, the patent application pending with the German Patent and Trademark Office under file number DE 10 2009 026 961 provides that a radio-frequency (RF) communication be carried out via an arbitrary bus system, for example a CAN bus, with an arbitrary access protocol. Here, the access to the second channel used by a plurality of subscriber stations for the RF communication can be controlled according to an arbitrary access method. First data that are to be transmitted via the first channel, and second data that are to be transmitted via the second channel, are transmitted via a joint signal line.